FIG. 1 shows a cross section of a typical conventional PM stepping motor of claw pole type (refer, for example, to Japanese Patent Application Laid-Open No. H4-222454, FIG. 9). The PM stepping motor (hereinafter referred to simply as “stepping motor” as appropriate) shown in FIG. 1 generally includes a stator assembly and a rotor assembly 130.
The stator assembly is composed essentially of two stator units which are axially coupled to each other by resin molding and which each include: an inner yoke 111 including a plurality of pole teeth 131; an outer yoke 122 including a plurality of pole teeth 123 and coupled to the inner yoke 111 with its pole teeth 123 intermeshing with the pole teeth 131 of the inner yoke 111 in the same circumferential plane; a bobbin 106 molded of resin so as to enclose the pole teeth 131 and 123 in a consolidated manner therewith and including inner and outer flanges 161; and a coil 121 wound around the bobbin 106, wherein a terminal block 162 is formed of resin so as to integrally bridge respective inner flanges 161 of the two stator units coupled to each other by resin molding.
A front plate 124 is attached to one side of the stator assembly structured as described above, and a rear plate 125 is attached to the other side thereof. A bearing 127 is attached to the center of the front plate 124, and a bearing 128 is attached to the center of the rear plate 125.
The rotor assembly 130 includes a shaft 129 and a magnet 132 magnetized circumferentially. The rotor assembly 130 is rotatably disposed in the hollow of the stator assembly such that the shaft 129 is rotatably supported by the bearings 127 and 128, wherein the outer circumferential surface of the magnet 132 opposes the pole teeth 131 and 123 of the inner and outer yokes 111 and 122 with a gap provided in between.
In the stepping motor shown in FIG. 1, while the two stator units of the stator assembly are axially coupled to each other by resin molding and therefore can be precisely aligned with each other, it is difficult to align the bearings 127 and the bearing 128 coaxially with respect to the stator units because the bearings 127 and 128 are attached respectively to the front and rear plates 124 and 125 which are individually attached to the respective sides of the stator assembly as described above. Consequently, the aforementioned gap, which is provided between the outer circumferential surface of the magnet 132 and the pole teeth 131 and 123 of the inner and outer yokes 111 and 122, and which is desired to be as small as possible, cannot be set as desired.
In order to overcome the above difficulty about coaxial alignment of the bearings 127 and 128 with respect to the stator units, a stepping motor is disclosed in which a bearing is press-fitted inside a hollow defined by pole teeth of an outer yoke (refer, for example, to Japanese Utility Model Patent Application Laid-Open No. H6-029388).
FIG. 2 shows a cross section (partly) of such a stepping motor as described above, and FIG. 3 shows a cross section of a bearing area enlarged of FIG. 2.
Referring to FIGS. 2 and 3, a brim portion 241b of a bearing 241 is press-fitted in a hollow defined by pole teeth 214a of an outer yoke 214, whereby the bearing 241 can be disposed in place with respect to the outer yoke 214 of a stator unit with an improved coaxial alignment therewith.
In the stepping motor of FIG. 2, however, the pole teeth 214a are formed by a bending process, which inevitably involves variation in bending condition. Accordingly, it is likely to happen that the brim portion 241b of the bearing 241, when press-fitted in the hollow defined by the pole teeth 214a of the outer yoke 214, makes contact with the pole teeth 214a with variation from unit to unit, and therefore it can happen that the bearing 241 is not retained firmly by the pole teeth 214a, or that the bearing 241 is tilted thus causing a gap between its rotor magnet and the pole teeth 214a to be uneven from place to place.